The present invention relates to an environmental radioactivity monitor, and in particular to an air monitor for monitoring for the presence of radioactive materials in the air, and a method of analyzing signals in such a monitor.
Such air monitors are currently known, consisting of a pump to pump a stream of air through a filter, and means to detect radiation emitted by particles trapped on the filter. A problem with all such devices is the need to discriminate against radiation from natural background sources, in particular a particles emitted by radon and thoron daughters. The principal background sources of xcex1 radiation are as follows:
Po-218 (RaA) at 6.0 MeV
Po-214 (RaCxe2x80x2) at 7.68 MeV
Bi-212 (ThC) at 6.05 MeV
Po-212 (ThCxe2x80x2) at 8.78 MeV.
The two thoron daughters, ThC and ThCxe2x80x2, are always in equilibrium with each other. It will be appreciated that the radiation from RaA and from ThC overlap with each other in the energy spectrum.
The principal radioactive nuclides of interest are typically isotopes of uranium, plutonium or americium that emit xcex1 rays of energy up to 5.5 MeV. For example:
U-234 (4.75 MeV)
U-235 (4.15 to 4.6 MeV)
U-238 (4.2 MeV)
Pu-238 (5.5 MeV)
Pu-239 (5.15 MeV)
Pu-240 (5.2 MeV)
Am-241 (5.5 MeV)
In each of these cases the a radiation is of lower energy than those emitted by the background nuclides. The xcex1 radiation from any such radioisotope is emitted with a well-defined energy, but the energy received and detected by the detector depends upon the path travelled by the xcex1 particle; consequently the peaks in the energy spectrum due to xcex1 radiation are asymmetrical, with a gradual approximately exponential decline on the low energy side of each peak that may be referred to as a low-energy tail.
Usually the background due to radon and thoron daughters"" xcex1 activity collected by a filter is considerably greater than the xcex1 activity of interest. For example it is often the case that the xcex1 background is several hundred times that corresponding to an alarm level of plutonium. One way such discrimination can be performed is to measure the count rate in a part of the spectrum in which only background radiation is expected, for example in an energy window between say 5.6 MeV and 6.3 MeV xcex1 energy, and then to assume that the background radiation in another part of the spectrum (where radiation from nuclides of interest would be detected) can be simply related to that in the first part of the spectrum. This approach may be improved to provide somewhat better discrimination by dividing the spectrum into say four windows that include different portions of signal xcex1 activity of interest, and the three background components, for example: 3 MeV to 5.6 MeV (signal plus all background nuclides); 5.6 MeV to 6.3 MeV (all background nuclides); 6.3 MeV to 8.0 MeV (RaCxe2x80x2, ThC+ThCxe2x80x2); and 8.0 MeV to 10 MeV (ThCxe2x80x2 only). Such an approach can be reasonably effective, but may become inaccurate as a result of changes in the shape and position of the peaks in the spectrum; these changes may for example arise from changes in aerosol size, dust loading on the filter, filter fibre characteristics, air density, and amplifier gain drifts. An aim of the present invention is to overcome such problems by taking changes of spectrum shape into account.
According to the present invention there is provided an environmental radioactivity monitor including a radiation detector to detect radiation emitted from a sample that may comprise radioactive material from the environment, wherein the monitor incorporates means to analyse the signals from the radiation detector into a multiplicity of energy channels, means to use the counts in at least two energy windows to determine the shape of the low-energy tail of the ThCxe2x80x2 peak, means to use the shape of the ThCxe2x80x2 peak to predict the ThC peak, and means to correct the energy spectrum by stripping out the low-energy tail of the ThCxe2x80x2 peak and stripping out the ThC peak; means to use the counts in at least two energy windows to determine the shape of the low-energy tail of the RaCxe2x80x2 peak, and means to correct the energy spectrum by stripping out the low-energy tail of the RaCxe2x80x2 peak.
The monitor having these features is suitable for detecting radioactive material in the sample, in the presence of sources of background radiation, as long as there is no significant peak from RaA. This would not generally be true for monitors making real-time observations, but would be true for samples separated from the environment for a few minutes, because RaA itself has a half life of about 3 minutes and is generated by radon (Rn-222) which is a gas. In situations where there may be a peak from RaA, such as an air monitor making real-time measurements, the monitor of the invention also incorporates means to use the shape of the RaCxe2x80x2 peak to predict the RaA peak, and means to correct the energy spectrum by stripping out the RaA peak.
Preferably the signals are analyzed using a multi-channel analyzer, this typically having 256 channels. These channels may be arranged to cover the energy range 0 to 10 MeV. The low end of this range is likely to primarily result from xcex2 radiation, and the signals of energy above say 2.5 MeV can be assumed to result from xcex1 particles. In determining the shapes of the low-energy tails, the energy windows would usually combine the counts from a number of adjacent channels. This can increase the counts being analyzed to determine the shape, so decreasing the response time. It will be appreciated that the fundamental requirements for an environmental radiation monitor are to provide accurate and reliable measurements within a short response time. The present invention enables each separate component of the background xcex1 radiation to be stripped individually, so that accurate results can be obtained even if spectral shapes and peak locations are changing, and even if the relative activities of the xcex1-emitting background nuclides are changing.
The monitor may also include means to locate the peak and upper edge of each component of the xcex1 background spectrum, so that if necessary any changes in these locations can be taken into account. The monitor preferably also comprises means to identify signals corresponding to xcex2 particles.
In a preferred embodiment the sample is a filter through which air has been caused to flow. For example the environmental radiation monitor may be an air monitor, that is to say it may comprise a filter, and means to cause air to flow through the filter, the radiation detector being arranged to detect radiation emitted from the filter.